1. Field of the Invention
This invention is in the field of pressure-sensitive adhesive compositions comprising a combination of synthetic and/or natural rubber in combination with a synthetic aromatic hydrocarbon resin resulting from the polymerization of a selected fraction of thermally cracked petroleum oil. The relative amounts of conjugated diolefins and indene and its derivatives are restricted to provide a highly satisfactory tackiness, adhesive and cohesive strength and other important properties.
2. Description of the Prior Art
Generally, pressure-sensitive adhesive compositions are considered satisfactory if they have a proper balance of tackiness, adhesive strength, cohesive strength, and weather resistance. Resinous materials which serve as adhesion and tack-imparting components must be highly compatible with rubbers and soluble in solvents.
Heretofore, pressure-sensitive adhesive compositions which have been used as adhesive coatings on tapes, sheets of paper, fabric and other backing materials have been proposed which are mixtures of natural rubber and/or synthetic rubber with a terpene resin. The resin is usually .alpha.-pinene, .beta.-pinene, or mixtures thereof which are present in refined pine resins. These naturally occurring substances are becoming more scarce and considerably more expensive.
Known pressure-sensitive adhesive compositions may contain rosin esters, aliphatic hydrocarbons, and cumarone-indene resins as tackifiers. These resins, however, are inferior to terpene resins with regard to their balance of cohesive strength, adhesive strength, tackiness and other important properties, and are further not very compatible with rubbers and are not well soluble in solvents.
Still another type of synthetic resin has been described in Douglas et al. U.S. Pat. No. 3,932,332. This patent discloses the manufacture of copolymers of .alpha.-methylstyrene and styrene in which the .alpha.-methylstyrene is present in amounts of 50 to 90% and the softening point ranges from about 60.degree. to 100.degree. C. This starting material may be commercially available but would have to be further purified at the expense of additional cost.